JPH0210374B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a technique for improving the durability of a gas sensor.

[Description of prior art]

Hydrogen gas sensors can be roughly divided into electrical types and optical types. Electric types use an oxide semiconductor layer such as SnO ₂ or ZnO on an insulating substrate.
It is also known to provide a pair of electrodes facing each other with a gap on the semiconductor layer, and measure the change in electrical resistance caused by the contact reaction of the semiconductor layer with hydrogen gas. In addition, as an explosion-proof all-optical type, an optical waveguide is provided on a transparent substrate, and an upper layer made of a substance that adsorbs and dissociates hydrogen, such as palladium (Pd), and the dissociated hydrogen generates electrons and protons. The sensor is constructed by providing a laminated film formed with a lower layer made of a substance whose light absorption coefficient changes in response to hydrogen, such as tungsten oxide, and the light absorption coefficient of the lower layer of the laminated film increases due to reaction with hydrogen, thereby guiding the light. There are sensors that utilize the phenomenon that the amount of light emitted from a wave path decreases.

[Problems with conventional technology]

In either type of gas sensor, the sensing part that comes into contact with the gas is exposed to the outside air, and the material used in the sensing part deteriorates due to contact with moisture, so the lifespan of the sensor is relatively short. There was a problem with the paddle.

[Means to solve conventional problems]

The gas detection section is formed of an adsorption layer made of a substance that adsorbs and dissociates hydrogen, and a light absorption layer provided below the layer and made of a substance whose light absorption coefficient changes in response to the dissociated hydrogen. The exposed surface is covered with a coating layer made of a non-hydrophilic continuous porous material. Tetrafluoroethylene resin is particularly suitable as the material for the above-mentioned coating layer, and other materials with good weather resistance such as polychlorotrifluoroethylene, vinyl fluoride, trifluoroethylene, vinylidene fluoride, and propylene hexafluoride are also suitable. A fluororesin can be used.

Further, the above-mentioned coating layer may be composed of an inorganic substance other than an organic resin.

[Effect]

Since the protective coating layer is made of a continuous porous material, it is breathable, allowing the atmosphere containing the gas to be detected to pass through freely and easily reach the gas detection part, so the gas detection characteristics are not affected at all. I don't accept it.

Furthermore, since the coating layer is non-hydrophilic, even if it is continuous porous, moisture does not permeate through it due to capillary action. Furthermore, the smaller the pore diameter, the higher the water pressure resistance and the better the waterproofness. Furthermore, when the coating layer is adhesively fixed to the sensor body, even a non-adhesive material such as fluororesin can be firmly fixed due to the anchoring effect caused by the adhesive penetrating into the pores.

〔Example〕

The present invention will be described in detail below based on embodiments shown in the drawings.

FIG. 1 shows an example in which the present invention is applied to a flat plate optical waveguide type hydrogen gas sensor, in which a sensor 1 is provided with an optical waveguide 3 having a higher refractive index than that of the substrate on a transparent dielectric substrate 2. A laminated film of a light absorption layer 5 and a hydrogen adsorption layer 6 is provided on the gas detection section 3 .

The adsorption layer 6 on the surface side is formed of a metal thin film such as palladium (Pd) or platinum (Pt) that adsorbs and dissociates hydrogen gas, and the light absorption layer 5 below the adsorption layer 6 adsorbs and dissociates hydrogen gas. It is made of a material, such as tungsten oxide (WO ₃ ), whose light absorption coefficient changes when it receives electrons and protons.

In the sensor having the above structure, an input optical fiber 7A and an output optical fiber 7B are connected to both ends of the optical waveguide 3, respectively, the other end of the input fiber 7A is connected to a light source 8 such as a laser, and the other end of the output fiber 7B is connected to are respectively connected to a light receiving detector 9 such as a PIN photodiode.

In the above sensor, when hydrogen gas comes into contact with the adsorption layer 6, electrons and protons are generated by the hydrogen reduction action of the layer, and these are injected into the light absorption layer 5 made of WO ₃ below, and the following reaction occurs. arise.

_{WO 3 +} xH ⁺ ₊ Attenuation of the evanescent wave passing through waveguide 5 increases, and the amount of light emitted from waveguide 3 decreases. By measuring this change in the amount of received light, the hydrogen concentration can be detected.

In the hydrogen gas sensor having the above structure, when the entire surface of the adsorption layer 6 or the adsorption layer 6 is partially provided on the surface of the absorption layer 5, the upper surface of the light absorption layer 5 is partially exposed to the outside air. Therefore, both these layers 5,
There is a problem that the exposed surface of No. 6 comes into contact with moisture and deteriorates.

Therefore, in the present invention, as shown in FIG. 1, the entire surfaces of both layers 5 and 6 forming the gas detection section 4 are covered with a protective coating layer 10 made of a non-hydrophilic continuous porous material. For example, the gas detection section 4 is provided approximately at the center of the surface of the substrate 2, a separately prepared continuous porous coating film 10 is placed over the gas detection section 4, and the peripheral portion of the film 10 is adhesively fixed to the upper surface of the substrate 2.

As an example, LiNbO ₃ is used as the substrate 2, Ti is thermally diffused onto the substrate 2 to form an optical waveguide 3, and a WO ₃ film is vacuum-deposited to a thickness of 1 μm on the waveguide 3 to form a light absorption layer. 5.

WO ₃ uses pellets with a purity of 99.99% and a W-wire crucible coated with alumina to produce 1×10 ^-5
Vacuum evaporation is performed at a substrate temperature of 100°C at a vacuum degree of less than Torr. At this temperature, WO ₃ becomes amorphous and has a blue color due to oxygen defects. The substrate with the WO ₃ film was heated for 200 min in an oxygen atmosphere.
It is stabilized by annealing at ℃ for 1 hour.

Next, Pd was deposited on the WO ₃ film as a hydrogen adsorption layer 6.
is deposited by sputtering to a thickness of 100 Å.
Further, as a protective coating layer 10, a polytetrafluoroethylene resin film which has been uniaxially or biaxially stretched to have a continuous porous structure is laminated on top of this, and is adhesively fixed at the outer periphery. With the above structure, the atmosphere containing hydrogen gas can freely pass through the coating layer 10, and the detection performance remains the same as before.

In addition, since the coating layer 10 is non-hydrophilic, even if the pore size is small, no moisture permeation occurs due to capillary action, and the smaller the pore size, the higher the water pressure resistance and the improved waterproofness.

The sensor with the above structure can measure a hydrogen gas concentration range of 10 to 2000 ppm with an accuracy of ±5%, and can maintain its initial performance over a long period of time. FIG. 2 shows a second embodiment of the invention. In this example, the optical waveguide 3 is formed of a material whose light absorption coefficient changes when receiving electrons and protons.
The structure is otherwise similar to that of the previous embodiment.

A third embodiment of the present invention is shown in FIGS. 3 and 4. This example is an example in which a focusing lens is used as the optical waveguide main body, and the main body 11 forming the optical waveguide is a well-known transparent cylindrical body with a distribution in which the refractive index is maximum on the central axis and gradually decreases toward the periphery. It consists of a semi-cylindrical body with a focusing lens divided in half at the center. The length is set as an integral multiple of 1/2 pitch of the meandering period of the light beam.

A light absorption layer 5 and an adsorption layer 6 are provided on the flat side surface 11A of the semi-cylindrical main body 11, and a coating layer 10 made of a non-hydrophilic continuous porous material covers both layers 5 and 6 entirely. are provided, and the input fiber 7A and the output fiber 7B are connected to both end surfaces of the main body 11, respectively.

In the sensor with the above structure, the light ray 12 that enters the main body 11 from the input fiber 7A draws a substantially sine curve and is directed toward the flat side surface 11A. The light is reflected at the interface with the absorption layer 5, travels inside the main body 11 again, and then enters the output fiber 7B.

FIG. 5 shows a fourth embodiment of the present invention.

In this example, an optical fiber 15 in which a high refractive index core portion 13 is covered with a low refractive index cladding layer 14 is used as the optical waveguide 3, and the cladding layer 14 of this fiber 15 is partially removed to expose the core portion 13. , a light absorption layer 5, a hydrogen adsorption layer 6, and a coating layer 1 of a continuous porous body surround this exposed core portion 13.
0 is set.

〔effect〕

According to the present invention, the gas detection section of the hydrogen gas sensor is protected from contact with moisture, and at the same time, the detection performance is not degraded by the protective coating, and stable performance can be maintained over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a side sectional view showing a first embodiment of the present invention;
FIG. 2 is a side sectional view showing a second embodiment of the invention, FIG. 3 is a side sectional view showing a third embodiment of the invention, and FIG.
This figure is a front view of the same, and FIG. 5 is a side sectional view showing a fourth embodiment of the present invention. 1... Gas sensor, 3... Optical waveguide, 4...
Gas detection unit, 5... Light absorption layer, 6... Hydrogen adsorption layer, 7A... Optical fiber for input, 7B... Optical fiber for output, 8... Light source, 9... Light receiving detector, 10... Continuous Covering layer of porous material.

Claims (1)

[Claims] 1. The gas detection section is formed of an adsorption layer made of a substance that adsorbs and dissociates hydrogen, and a light absorption layer provided below the layer and made of a substance whose light absorption coefficient changes in response to the dissociated hydrogen, A hydrogen gas sensor characterized in that the exposed surface of the layer is covered with a coating layer made of a non-hydrophilic continuous porous material.